1. Field of the Invention
The present invention is directed to an LED (light emitting diode) and reflector illumination device that creates a highly uniform illumination/intensity pattern.
2. Description of the Related Art
In many applications it is desirable to create a uniform illumination pattern used for general illumination applications such as high-bay, low-bay, parking area, warehouses, street lighting, parking garage lighting, and walkway lighting. In these applications the light fixture must direct the majority of the light outward at high angles and have only a small percentage of the light directed downward.
Generally, light sources emit light in a spherical pattern. Light emitting diodes (LEDs) are unique in that they emit light into a hemispherical pattern from about −90° to 90° as shown in FIG. 10A. Therefore, to utilize an LED as a light source in a conventional manner reflectors are placed around an LED.
When a light source illuminates a planar target surface area directly in front of it, as is the case when the LED optical axis is aligned to the light fixture optical axis, the illuminance in footcandles (fc) decreases as a function of the Cos3 θ. This is known as the Cos3 θ effect. The LED distribution shown in FIG. 10A approximately follows a Cos θ distribution. A Cos4 θ illumination profile results when a light source with a Cos θ intensity distribution illuminates a surface due to the combination of the Cos θ and the Cos3 θ effect. The Cos4 θ illumination distribution would result in front of the LED if no optic is used with a typical LED source. FIG. 10B illustrates this by showing the high illuminance level at a value of 0 for the ratio of distance to mounting height (directly below the fixture) for the background LED illumination device with no optic. The illuminance values drop off rapidly and reach almost 0 at a value of 2.5 for the ratio of distance to mounting height.
FIG. 11 shows a background LED illumination device 10 including an LED 1 and a reflector 11. The reflector 11 can revolve around the LED 1. In the background LED illumination device in FIG. 11 the LED 1 and reflector 11 are oriented along the same axis 12, i.e. along a central optical axis 12 of the reflector 11, and the LED 1 points directly out of the reflector 11 along the axis 12.
With the LED illumination device 10 in FIG. 11, wide-angle light is redirected off of the reflector 11 and narrow angle light directly escapes. The result is that the output of the LED illumination device 10 is a narrower and more collimated beam of light. Thereby, with such an LED illumination device 10, a circular-based illumination pattern is created. Since most LEDs have a Cosine-like intensity pattern as shown in FIG. 10a, this results in a hot spot directly in front of the LEDs when illuminating a target surface. The reflector 11 can increase the illuminance at various areas of the target surface but the reflector 11 cannot reduce the hot spot directly in front of the LED 1.
Therefore, orienting the LED 1 and the reflector 11 along the same axis 12 as in FIG. 11 while pointing the LED 1 directly toward a target area, such as downward toward the ground, results in a hot spot directly in front of the light fixture.